Need Help With Libinput

I desire a relatively slow pointing device (i.e. mouse) motion,
but with the now mandatory (i.e. no choice) libinput driver
under X Window I have a hard time understanding how to configure
the motion.

Using "xinput list" I discover that my pointing device is
device id=8.

Thus:

xinput list-props 8

...
...
Device 'PixArt Microsoft USB Optical Mouse':
        Device Enabled (155):   1
        Coordinate Transformation Matrix (157): 0.400000, 0.000000, 0.000000, 0.000000, 0.400000, 0.000000, 0.000000, 0.000000, 1.000000
...
...


The motion is apparently controlled by this "transformation
matrix," which seems to be 3x3 in size.

As seen above, I have what I assume to be the x and y motion
set to 0.4.

This does indeed slow the motion down.

But what do the rest of the values accomplish?

I can find no documentation on this issue.  If someone can
provide links or a description it would be appreciated.